In bonding electronic devices, lead-free solders have been wanted, but as to high temperature solders, there have been no substitute materials so far. In encapsulating semiconductor devices, it is necessary to use hierarchical solders, and substitute for the high temperature solder materials have been demanded.
If a particle size of metal particles for substitutes of the high temperature solders becomes 100 nm or less, wherein the number of atoms is small, a specific surface area of the particle per a volume of the particles drastically increases, and it has been known that a melting point and a sintering temperature thereof will remarkably lower, compared to those of bulk materials.
By virtue of this low temperature sintering effect, a bonding material using metal particles having a particle size of 1 to 100 nm has been studied. See for example patent document No. 1: Japanese patent laid-open 2004-107728 (US 2004/024564). The patent document No. 1 discloses a bonding material wherein nuclei of metal particles having an average particle size of 100 nm or less are covered with an organic coating and the coated particles are dispersed in a liquid, and the bonding is carried out by sintering the metal particles by heating. Since in this bonding method the metal particles after bonding change into bulk metal and metallic bonding is effected at interfaces between the metal particles, the bonding gives high heat resistance and reliability and high heat dissipation efficiency. In the patent document No. 1, the above bonding materials are referred to as a substitute for the high temperature solders that do not contain lead.
However, if the paste of the sintering type, which contains the metal particles having the average particle size of 100 nm or less as a main ingredient for the bonding material is used, it has been elucidated that while a sufficient bonding strength is obtained in cases of noble metals such as Au, Ag, Pd etc, it is impossible to bond semiconductors made of Si or SiC, which have oxide films on faces for bonding. FIG. 6 shows evaluation results of bonding strengths with respect to various bonding members. A bonding temperature was 300° C. and a bonding pressure was 1.0 MPa constant, a bonding material silver particles having an average particle size of 10 nm, coated with an amine series organic material. A bonding atmosphere was air. Bonding members to be bonded were Au, Ag, Si and SiC.
The vertical line in FIG. 6 indicates a shearing strength at the bonded face, wherein a shearing strength of silver metal is one and the shear strengths at the bonding faces were a ratio to that of the silver metal. As is apparent from the results, in case of bonding in air, the sintering type bonding materials show good bonding characteristics with respect to such metals as Au, Ag, etc that are hard to be oxidized, but they do not show any bondability to Si and SiC at all. This is because an oxidation film exists on a bonding face (rear side face) of Si series semiconductor elements, and hence the oxidation film does not show the bonding characteristics at the time of sintering of the metal particles. Therefore, it has been common to form a metalized film on the rear side face of the semiconductor elements so that the bonding is effected between the semiconductor elements and supporting substrates by means of the metalized film.
Alcohol series organic materials for covering the ultra fine particles disclosed in patent document No. 1 vanish only by heating in air, which are useful for not oxidizable noble metals, but may not be useful for Si series materials having an oxide film on its surface.
In the patent document No. 1, the ultra fine metal particles are used as a starting material in a form of metal salts such as organic acid salts of metals, wherein the metal salts and organic materials such as alcohol are dispersed in an organic solvent to form a paste, and the mixture is heated to thereby form the ultra fine metal particles resulting in composite metal particles, which combine with the organic materials.
Patent document No. 2 (Japanese patent laid-open 2008-178911) discloses a bonding material comprising particles of a metal oxide, metal carbonate salt or metal carboxylic salt having an average particle size of 1 to 50 nm, and an organic reducing agent, the particles being mixed with the reducing agent, but the patent document does not disclose bonding of a substrate and a semiconductor element via an oxide film.
Although US 2009/0244868, which is not a prior art and published after the present patent application in Japan discloses a bonding structure wherein an aluminum electrode and of a semiconductor and a silver bonding layer are bonded via an amorphous layer (aluminum oxide film). The bonding layer, which comprises particles of a metal oxide, a metal acetate or a metal formic acid compound having a particle size of 1 to 50 nm and an organic reducing agent, is prepared by heating the bonding material, but it does not disclose a structure wherein a semiconductor element and a supporting substrate are bonded via a Si oxide film present in a rear face of the Si type semiconductor element.